The support tubes or guide tubes, which are longer than the fuel rods, in cooperation with two transverse end plates, assure retention of the grids, the rigidity of the assembly, guiding of the control rods and taking up of axial forces.
In the parts of the guide tubes between the upper tip and the upper end grid and also between the lower tip and the lower end grid, axial forces are transmitted only by the guide tubes, while in the whole of the general region of the assembly located between the end grids, axial force is also transmitted by the rods.
The core in pressurized water nuclear reactors, for example, is constituted by such very long, square-sectioned assemblies.
The end plates of such assemblies are generally made of stainless steel, while the spacer grids disposed at regular intervals along the length of the assembly are generally made of a nickel alloy or a zirconium alloy.
In practice, these materials have adequate mechanical and physical properties for constituting these parts of the assembly.
In known embodiments, provision is made to fix all the guide tubes to each spacer grid so that each guide tube shares in the taking up of axial forces.
When the guide tubes and the spacer grids are stainless steel, the various parts, i.e., the end plates, the spacer grids and the guide tubes, constituting the framework of the assembly inside which the fuel rods are positioned, are easily connected by electrical welding.
Materials such as stainless steel or nickel alloys, however, are materials with high neutron capture cross sections, so that the performance of fuel assemblies including a significant mass of these materials is proportionately reduced.
The use of guide tubes made of a material with a low neutron capture cross section, such as a zirconium alloy, has therefore been proposed, e.g., in French Pat. No. 2,049,108.
The zirconium alloys used, however, cannot be welded to stainless steel and the nickel alloys respectively constituting the end plates and the assembly grids.
The French patent therefore proposes using intermediate connecting apparatuses welded onto the end plates and onto the grids and effecting mechanical attachment between the guide tubes made of zirconium alloy (Zircaloy) and these intermediate connecting parts. The latter can be sleeves made of stainless steel, for example, into which the tubes made of zirconium alloy are introduced and fitted securely.
Such an assembly includes a smaller mass of material with a high neutron capture cross section in relation to previously known assemblies, but many complex operations are involved in manufacturing it, since stainless steel sleeves must be welded into each of the cells of the spacer grids inside which the guide tubes pass, guide tubes must be introduced into these sleeves and then guide tube must be fixed in the sleeve by introducing tooling inside the guide tubes.
In addition, under irradiation, zirconium alloy tubes enlarge and expand less than the steel parts generally used for end parts, so that provision must be made in designing the assembly for this possibility of differential lengthening of the assembly in service.
The object of the invention is therefore to propose a fuel assembly for a nuclear reactor constituted by a bundle of parallel fuel rods whose spacing is maintained by spacer grids which are transverse with respect to the rods, two traverse end plates and supporting guide tubes which replace some fuel rods, and which are longer than fuel rods, rigidly fixed to the end plates and to the spacer grids, assuring retention of the grids, rigidity of the assembly, and taking up of axial forces as well as guiding of the control rods of the reactor, the end plates being made of a material with a high neutron capture cross section, such as stainless steel, this fuel assembly having a structure which allows easier and cheaper manufacture and assures adequate resistance to axial forces.